FIG. 1 shows a schematic diagram of a conventional power transfer device 100. The power transfer device 100 includes a power transfer switch Q1 (or a load switch), an activating unit 102, and a switch control unit 104. The power transfer switch Q1 includes an input terminal VA and an output terminal VA_S. The power transfer switch Q1 controls a connection between a power source (not shown) coupled to the input terminal VA and a load (not shown) coupled to the output terminal VA_S, and transfers power from the power source to the load. The activating unit 102 and the switch control unit 104 control a state of the power transfer switch Q1. To be specific, if an activating signal AS generated by the activating unit 102 is at logic high, then the switch Q2 is on and the switch Q3 is off, and therefore the power supply VCC charges the capacitor C1 to increase a gate voltage of the power transfer switch Q1 to gradually turn on the power transfer switch Q1. This process can be referred to as a “start-up process.” When the power transfer switch Q1 is fully turned on (e.g., the voltages at the terminals VA and VA_S are substantially the same), the start-up process ends. If the activating signal AS generated by the activating unit 102 is at logic low, then the switch Q2 is off and the switch Q3 is on, and therefore the capacitor C1 discharges to ground to decrease the gate voltage of the power transfer switch Q1 to turn off the power transfer switch Q1.
The power transfer device 100 may be problematic. By way of example, on one hand, in order to maintain the load to receive power through the power transfer switch Q1, the current path of the series-coupled resistor R2 and switch Q2 needs to remain on to allow a current to pass through. On the other hand, in order to maintain the load disconnected from the power source, the current path of the series-coupled resistor R3 and switch Q3 needs to remain on to allow a current to pass through. This may lead to relatively high power consumption by the power transfer device 100.
Furthermore, the time it takes for the voltage at the output terminal VA_S to increase from a ground voltage to the voltage at the input terminal VA can be referred to as “start-up time.” Different load devices may require different start-up times. For example, a load device may require a shorter start-up time to start up quickly, while another load device may require a longer start-up time to start up slowly. As another example, a load device may need its supply voltage (e.g., the voltage at the output terminal VA_S) to be controllable (e.g., to increase steadily) during the start-up process. However, once the power transfer device 100 is designed and built, it may not be able to satisfy all of these different requirements and so it may not be used with some types of load devices.